This invention relates to a power transmission mechanism and, more particularly to an improved power transmission mechanism of the kind comprising an hourglass worm and a worm wheel.
In the case of worm gearing consisting of an hourglass worm and a worm wheel, a large number of teeth on the worm wheel are meshed with a continuous helical tooth formed on the hourglass worm and the area supporting the pressure between the teeth of the worm wheel and the tooth of the worm is greater than in the case of the more usual worm gearing with a cylindrical worm, and it has, therefore, advantages that the wear of the teeth is reduced and large forces can be transmitted with a comparatively small driving force.
However, this kind of the worm gearing has a disadvantage in that when the worm is moved slightly in a direction which is at right angles to the longitudinal axis of the worm something goes wrong with the engagement of the hourglass worm and worm wheel.
To improve conventional power transmission mechanisms including an hourglass worm, the inventor of the present invention has already proposed that a power transmission mechanism comprising an hourglass worm, a wheel device having a plurality of recesses therein spaced around the circumference thereof, the spacing between the recesses corresponding to the pitch of a continuous tooth groove in the hourglass worm, and ball or roller means mounted in meshing engagement with the groove and each ball or roller means being rotated at least about an axis which extends substantially in the direction of rotation of the wheel device.
According to the above, the wheel device comprises a pair of disks so that the ball or roller means can easily be mounted on the wheel device. Each of the disks is provided with a number of slots on the circumference of the disks at equal distance with the pitch of the tooth groove in the worm. This, however, has the disadvantage that it is difficult to manufacture.
In the above proposed power transmission mechanism, each slot is formed to incline at a desired lead angle with respect to a plane parallel to the surface of the disc and a ball bearing unit is mounted in each slot. This requires an extremely precise process.